In accordance with recent increase in communication capacity, systems using optical signals are becoming widespread, which arises an important issue to ensure the reliability of optical devices used in the systems and which demands a technique of predicting a possible failure of the optical devices.
Conventionally, typical detection of a prospective failure of a semiconductor laser (Laser Diode, LD) has used a Photo Detector (PD). Specifically, part of light emitted from a semiconductor laser is received by a PD, which detects a degree of deterioration in optical power to predict a prospective failure. However, the above scheme using a PD is accomplished by an exterior circuit on which the PD is optically mounted, thereby increasing the costs.
For the above, there has been proposed a method of detecting a failure of a semiconductor laser by measuring the differential resistance of the semiconductor (e.g., see Patent Literature 1). The technique of Patent Literature 1 compares a differential resistance obtained by monitoring and a differential resistance threshold for judging a failure in order to judge a prospective failure (deterioration).    Patent Literature 1: Japanese Laid-open Patent Publication No. 2009-26889
However, the technique of Patent Literature 1 unstably measures a differential resistance of a semiconductor laser, which may result in a large variation of the measured differential resistance. In addition, a differential resistance varies for each individual semiconductor laser, which requires a modification of a value of the judgment.